The present invention relates to an ultrasound imaging system, and more particularly, to an ultrasound imaging system having an efficient hardware structure and capable of providing a high-resolution ultrasound image by adopting a multi-stage pulse compression scheme.
The ultrasound imaging system is widely used in the medical field for the purpose of displaying a sliced image (ultrasound image) of a xe2x80x9ctarget objectxe2x80x9d such as an internal organ of a human body. In such ultrasound imaging systems, an ultrasound image is formed by transmitting ultrasound signals towards the target object, receiving the signals reflected from the target object, more specifically, from a surface of the target object (e.g., an interface between skin and subcutaneous fat, between subcutaneous fat and abdominal muscles, etc., where the acoustic impedance is discontinuous), and converting the received ultrasound signals into electrical signals. For ultrasound signal transmission purposes, the ultrasound imaging system uses a transducer and a pulser for driving the transducer. The transducer generates ultrasound signals in response to a pulse applied from the pulser.
Most of the conventional state of the art ultrasound imaging systems employ a short pulse as ultrasound transmission signals. In such systems, the power of the signals received at the transducer is remarkably lowered since the transmitted ultrasound signal undergoes severe attenuation when passing through a highly dense medium, such as the human body. As a result, obtaining the desired information on the target object, e.g., in cases where the target object is located deep inside a body, becomes difficult. Increasing the peak voltage of the pulse being transmitted may solve the problems associated with the attenuation of the ultrasound signals. However, there is a certain limitation to increasing the peak voltage of the pulse, since this may affect the internal organs of the human body.
Instead of increasing the peak voltage of the pulse, the average power of the pulses can be raised. As a result, the Signal to Noise Ratio (SNR) can be improved remarkably. This method is called xe2x80x9cpulse compression,xe2x80x9d and is used, for example, in radar equipment. An ultrasound imaging system of the type that employs pulse compression normally uses a coded long pulse having a long duration instead of the conventional short pulse. In this type of ultrasound imaging systems, the resolution in the direction of ultrasound wave propagation, the xe2x80x9caxial resolution,xe2x80x9d is determined by the convolution taken between the characteristic function of the transducer and the coded long pulse, in contrast to a conventional system employing short pulses of high peak voltage where the axial resolution is determined by the impulse response characteristics of the transducer. Therefore, in order to avoid the degradation of the axial resolution that may be caused by the use of the coded long pulse, such ultrasound imaging apparatuses use a correlator-based pulse compressor that takes a cross-correlation between the received ultrasound signal and the coded long pulse as transmitted. Using the correlation at the pulse compressor can prevent degradation of the axial resolution, allowing the same level of resolution to be maintained as if a short pulse were transmitted. Accordingly, a relatively low voltage of the long duration can be advantageously used without sacrificing the SNR.
Additionally known in the art, ultrasound imaging systems may also be based on a phased array. Such an ultrasound imaging system includes a plurality of channels, each channel including a transducer, a transmitter (i.e., pulser) and a receiver coupled to the transducer. The transmitter functions to transmit ultrasound signals (or pulses) towards the target object such as a human body. Note that the transmitters at the plurality of channels do not transmit ultrasound signals at the same time. Instead, they transmit the ultrasound signals with a different timing so that the ultrasound signals as transmitted from the transmitters reach a desired position within the target object at the same time, thereby being transmit-focused at a predetermined location within the target object. The transmitted ultrasound pulses pass through various internal organs of the human body and are reflected from a certain portion of the internal organs and directed to the transducer array.
The ultrasound signals reflected from the target object are received by the transducer array and are converted into electric signals. The time when the reflected signals reach each of the transducers varies depending on the location of each transducer in the array relative to the target object. That is, the farther away from the center position of the array the transducer is located, the more time period is required for the ultrasound signals to reach the transducer. In order to compensate for the differences in arrival time among the transducers, a beamformer is used to receive focus the converted electrical signals. The beamformer incorporates appropriate time delays into the electrical signals, which correspond to the received ultrasound signals, giving rise to the same effects as if all the transducers receive the reflected signals at the same time. The time delays as applied by the beamformer vary depending on the depth of the reflecting surface of the target object and the locations of the transducers.
The beamformer is further explained below with reference to FIG. 1, which illustrates the structure of a beamformer in a conventional ultrasound imaging system. As shown, beamformer 100 comprises transducer array 10 including a plurality of transducers, delay stage 11 comprised of a corresponding number of delay elements DLY1-DLY64 to the transducers, adder 12, and pulse compressor 13 connected to the output terminal of adder 12. The reflected ultrasound signals are converted to electric signals at the transducers and are transmitted to delay stage 11. Each delay element at delay stage 11 compensates the input signals by a predetermined time delay depending on the location of the corresponding transducer relative to the center of transducer array 10. Therefore, the differences in arrival time among the transducers can be compensated by the use of delay elements, which are connected to the output terminals of the transducers. The delay-processed signals from delay elements DLY1-DLY64 are added together in adder 12. Pulse compressor 13 pulse-compresses the output signal from adder 12. According to the beamformer of FIG. 1, the system configuration can be simplified, but problems arise where the beamformer of FIG. 1 adopts dynamic receive-focusing. If receive-focusing is performed dynamically in the beamformer of FIG. 1 where pulse compression occurs after receive-focusing, delay times necessary for receive-focusing may be inaccurately computed, as explained below.
Preferably, the beamformer of FIG. 1 may adopt dynamic receive focusing, according to which a focusing point is dynamically changed while the ultrasound signals are propagating through the human body. According to the dynamic receive-focusing, the time delay value for the center transducer is fixed to a predetermined value. For some transducers adjacent to the center transducer, the time delay is controlled to be shorter than the fixed time delay for the center transducer. For the remaining transducers far from the center transducer, the time delay is controlled to be close to the fixed time delay of the center transducer. With the dynamic receive-focusing, the time delays for the transducers are continuously controlled to ensure that the signals reflected from the same focusing point can be summed. As a result, the time delays for the outside transducers transition from a low to high value, as the receive-focusing operation proceeds. Therefore, the ultrasound signals from the outside transducers are distorted as if their frequencies were lowered.
Turning again to the problems encountered with the beamformer of FIG. 1, the center transducer incurs no problem with the calculation of the delay time value even under the dynamic receive-focusing situation, since there is no time distortion in the ultrasound signals arriving at the center transducer. However, in the case of the outside transducers other than the center transducer, time distortion is inevitably caused by the dynamic receive-focusing so that pulse compressor 13 inevitably performs incomplete pulse compression. Because of the incomplete pulse compression, side lobes appear at an undesired position in the axial direction or the main lobes become wider, thereby degrading the ultrasound image quality. Particularly, the time distortion problem becomes severe when the focusing point is located near the transducers.
In order to solve the aforementioned problems, a slightly different beamformer arrangement from that of FIG. 1 is proposed which is shown in FIG. 2, in which one pulse compressor is connected to each channel of the transducer array. As shown, beamformer 200 comprises transducer array 15 having 64 transducers, pulse compressor section 16 having 64 pulse compressors PC1-PC64 each connected to the respective transducer, delay section 17 having 64 delay elements DLY1-DLY64 each connected to the respective pulse compressor, and adder 18. While one pulse compressor 13 is connected to the output terminal of adder 12 in FIG. 1, pulse compressors PC1-PC64 are connected between the transducers and delay elements DLY1-DLY64, according to the beamformer structure shown in FIG. 2. Therefore, pulse compressor section 16 of FIG. 2 pulse-compresses the output signals from the transducers before dynamically receive-focusing or variable-delaying them in delay section 17, thereby preventing an inaccurate calculation. However, the system of FIG. 2 has drawbacks in that it must include many pulse compressors with complex hardware structure, making the entire system configuration complicated and limiting the degree of integration at the hardware level. The complexity of the system further increases as the number of probes and transducers required for a particular application increases. Moreover, the frame rate is degraded due to the delaying and pulse compression operations that are needed for every channel.
It is, therefore, an objective of the present invention to provide an improved ultrasound imaging system that can reduce the necessary hardware size while providing a high-resolution ultrasound image.
According to one aspect of the present invention, an ultrasound imaging system for forming an ultrasound image is provided which comprises a transducer array, N number of first delay groups, N number of first adders, N number of pulse compressors, N number of second delay elements, and at least one second adder, wherein N is an integer more than 1. The transducer array is divided into N number of transducer groups. Each of the transducer groups includes M number of transducers, wherein M is an integer more than 1. The transducers function to convert incoming ultrasound signals into electric signals. Each of the N number of first delay groups includes M number of first delay elements wherein the first delay elements are connected to the respective transducers to delay the electric signals from the transducers by a first time delay. Each of the N number of first adders operates to add the M number of delayed signals from the respective first delay group. The N number of pulse compressors are respectively connected to the N number of first adders for pulse-compressing the added signals from the first adders. The N number of second delay elements are respectively connected to the N number of pulse compressors for delaying the pulse-compressed signals by a second time delay. The at least one second adder adds the delayed signals from the second delay elements to generate a receive-focused signal.
According to another aspect of the present invention, an ultrasound imaging method for forming an ultrasound image is provided which comprises the steps of transmitting ultrasound signals towards a target object using a transducer array, the transmitted ultrasound signals being reflected from the target object, receiving the reflected ultrasound signals, dividing the received ultrasound signals into N number of groups, and time-delaying and pulse-compressing the signals from the N number of groups.